How does Blender lights work in Cycles regarding intensity?
The light energy in Cycles are defined using the following unit: According to the doc: "Sun lamps are specified in Watts/m^2, other lamps are specified in Watts, and Emission shaders on meshes are also in Watts/m^2. I am not sure how the world energy is defined.

Because the energy are specified in watts (over an area when applicable), I don't see any reason why a perfectly diffuse surface should reflect the same numerical RGB value as the lamp's energy.